Due to many factors including the high cost of labor associated with populating circuit boards with components, technology in the electronic manufacturing arts has developed to automatically place such components on boards. The component placement art has developed to a point where a board may be populated with hundreds of components in a matter of seconds and simultaneously electrically interconnected in a desired fashion by well-known wave solder techniques or the like, as in the case of the emerging surface mounted component technology.
Whereas the component placement equipment and soldering systems have improved tremendously contributing to a rapid throughput in the overall manufacturing process for electronic boards or cards and the like, it has recently been appreciated that weak links or bottlenecks in the manufacturing chain can effectively reduce or eliminate the net gains achieved by improvements in other areas of the process such as the aforementioned advances in soldering and component placement techniques.
One such area recently recognized as having a great impact on the overall process is in the application of adhesives on the electronic cards or boards for purposes of affixing the components thereto during the component placement operation. This technology has become particularly important with the emergence of surface mounted components lacking any intrinsic means of holding themselves in place in contrast with conventional through-hole components held in place by built-in lead preforming, clinching, swagging, or the like.
In a typical application of such surface mount technology, wherein the surface mounted components are wave soldered to the bottom of a board, the components are held by adhesive to the bottom of the board during the soldering period. In some applications such as the reflow soldering process, the surface mount components are retained in position on the top of the board until completion of the soldering by means of an adhesive. Due to several factors such as slight asymmetry or irregularity in the solder lands to which the components are to be soldered, vibration, sliding, tombstoning or floating, the need became apparent for a means of placing accurate and reliable dot patterns of adhesives on the planar surface of the boards during the manufacturing process. Moreover, it further became evident that if such a process step was not to become a bottleneck in the overall electronic card/board manufacturing process, it was necessary to provide a system to place such dots not only reliably but extremely rapidly so as to keep pace with the emerging simultaneous component placement and soldering techniques.
Accordingly, the conventional prior art means for adhesive transfer rapidly became woefully inadequate, namely various apparatus having pressure syringes for depositing adhesive dots. Drawbacks of such apparatus included large system size and mechanical complexity rendering the system difficult to clean and maintain but, more importantly, limitation on the number of dots which might be placed simultaneously to relatively few.
Such limitations gave rise to refinement of an old art, namely screen printing, for this new application. Whereas such a system was relatively simple and provided for ease of cleaning and maintenance as well as simultaneous dot placement and uniform dot size, numerous disadvantages were nevertheless present. The most serious of these was the requirement that the planar surface presented to the screen printing process be substantially flat with no obstructions. However, this was impractical due to the requirements of modern electronic design which typically included boards already pre-populated with other components often of varying heights prior to commencement of the surface mount process, as, for example, in the case of pre-population with pin and hole components by a different process or other components such as trim pots, variable capacitors, or the like having different vertical dimensions from standardized SMT components or the like. Related to this problem was the conventional practice of populating both sides of a board rendering it difficult to support the unpopulated flat surface of one side of the board presented to the screening process when the opposing side was pre-populated with the aforementioned components of varying heights.
Such difficulties and disadvantages with the screen printing technique gave rise to yet a third method of adhesive application known as the pin-transfer process. In such a process a pin plate or pin array is fabricated for each printed circuit board or substrate to be processed. Pins are positioned in a plane on the plate in locations corresponding to the desired locations of adhesive dots on the work piece. During the process, the thus-fabricated pin plate is momentarily positioned so that the pin ends lightly contact a controlled thin layer of adhesive whereupon drops of the adhesive adhere to the pin ends. The pin plate and board are thence urged toward one another in any manner of mechanisms to effect momentary contact of the plurality of pins with the planar surface of the board. Upon withdrawing the pin board, adhesive dots are thereby deposited on the planar surface of the board in the desired pattern corresponding to the predetermined spatial relationship of the pins on the pin board and, of course, the desired spatial positioning of the surface mount components to be affixed to the board.
Such pin transfer systems provided numerous advantages. The process was relatively simple with little maintenance and provided ease of control of the adhesive quantity and the like. However, there were nevertheless numerous serious disadvantages presented by such a system. First, notwithstanding advances in automated tooling and fabricating machines such as CAD-CAM equipment, fabrication of pin plates was nevertheless costly and time consuming. It will be appreciated that a new pin plate was required for every variation of component makeup and positioning associated with different circuit board products. Thus, for example, design modification required the machining or etching of an entire new pin board or set of such boards to form pins in the desired pattern. Secondly, whereas the aforementioned pressure syringe system was adapted in some cases to accommodate non-flat systems, the pin transfer systems suffered the same drawback of the screen printing techniques, namely that they were primarily adapted for accommodating flat surfaces. One reason for this was that the pins, being fashioned of a metal are substantially non-compliant and thus, for example, could not accommodate curved or irregular surfaces such as those typically presented by slightly warped boards common in the art or larger boards by means of increasing the pressure with which the pins and the circuit board were brought into contact. Such inability to accommodate irregular surfaces resulted in non-uniform dot placement and, in the worst of cases, failure to place some adhesive dots at all. Yet additional drawbacks of pin transfer systems known in the art included manual steps required in the process which accordingly did not lend itself to rapid, modern production line techniques.
Due to the large numbers of extremely small components capable of being rapidly and simultaneously placed on boards and electrically connected thereto, it is readily apparent that an adhesion transfer system was highly desired which provided for extremely accurate and simultaneous adhesive pattern placement. Moreover, such a system was highly desired which readily accommodated easy and economical changes in the desired spatial positioning of the adhesive pattern to accommodate different products. Moreover, such a system was desired which further could accommodate wide variances in the characteristics of the surface presented for adhesive deposition in terms of irregularities including those associated with warpage, large boards, and boards pre-populated with other components, as well as the problems well known in the art associated with pre-population of boards on the opposing side to that which was to receive the adhesive transfer.